The present invention relates to a method for inspecting the quality of a modifier for vinyl chloride type resins and, more particularly, to a method for inspecting the quality of a modifier for vinyl chloride type resins which makes it possible to correctly and simply determine the number of fish eyes utilizing widely used apparatuses to perform quality inspection, which is particularly effective when such a modifier for vinyl chloride type resins is used in extrusion molding.
Vinyl chloride type resins are widely used and can be prepared at a low cost. Films, sheets or other various molded products thereof are excellent in a variety of properties such as transparency, resistance to oxygen permeation and printability and, thereof, they have been used as a container or packaging materials as well as vinyl films for agricultural purposes in large quantities.
One of conditions required for the films or sheets thus consumed in large quantities to be satisfied in order that they might be evaluated to have high quality and high commercial value is that they should have a number of fish eyes as low as possible. In most of cases, films and sheets having a lot of fish eyes frequently have so many fatal defects that these are not acceptable as articles of trade since they show a variety of drawbacks due to such defects. For instance, they not only have poor appearance and hence low commercial value but also are inferior in physical properties, and they cause tearing or cracking during fabrication thereof in the form of films and sheets such as vacuum molding and stretching treatment whereby the yield of final products is greatly reduced.
The inventors of the present invention have investigated the causes of the foregoing defects by inspecting and analyzing fish eyes and have found that in products having a lot of fish eyes, there are observed a large amount of non-gelled substances of those added as modifiers such as impact strength modifier (e.g., butadiene-styrene-methyl methacrylate resin (MBS resin)) and acrylic processing aids in addition to contaminants such as fibers and dusts, contaminants which are burnt due to thermal processings and non-gelled vinyl chloride resin as a starting material or the like.
In general, various modifiers such as the aforementioned impact strength modifiers and the processing aids are incorporated into rigid and semi-rigid films and sheets of vinyl chloride resin. In particular, it is almost inevitable to widely use these modifiers in films and sheets produced by blow molding, rigid calendering and T-die extrusion techniques. The amount thereof added to the vinyl chloride resin varies depending on required properties of the final products, but generally it ranges from 5 to 15 parts by weight per 100 parts by weight of vinyl chloride resin (hereinafter referred to as "PHR") for MBS resins, and 0.5 to 5 PHR for acrylic processing aids. The probability of causing fish eyes becomes correspondingly high.
To deal with this problem, as a part of quality control of starting resins, it is necessary to previously perform the quality inspection on the resins. Regarding the vinyl chloride resins, the method for inspecting quality disclosed in Japanese Patent Publication No. 61-40540 by the present applicant or the like may be used. On the other hand, the problem of contamination with foreign substances can be solved, for instance, by the preparation of good environment in manufacturing processes or by improving manners of preventing contamination through the use of bags, clothes and gloves. However, to date, there have not yet been developed any efficient, simple and correct methods for inspecting the quality of added resins such as impact strength modifiers and processing aids.
For instance, as a means for detecting or inspecting fish eyes which have generally been adopted presently, there has been known a method comprising extruding a vinyl chloride type resin compound as a T-die or inflation film utilizing a small-sized extruder provided with a screw having a diameter of 20 to 65 mm and determining the number of fish eyes present in the extruded film. In this case, much time and a lot of analyzing apparatuses are required to correctly investigate the cause of the generation of fish eyes. More specifically, in this method, fish eyes formed due to contaminants may approximately be distinguished from others with a loupe having a magnification of .times.50. However, regarding transparent particles or colored but transparent particles, it is required to first carry out chlorine analysis of the particles by an X-ray microanalyzer to thus determine whether these particles are formed due to vinyl chloride resins or not. Then, the particle is treated with osmium oxide and is subjected to analysis with a conventional electron microscope or an X-ray microanalyzer to confirm whether the particle is an MBS resin or not. On the other hand, processing aids have in general qualitatively been analyzed by a combustion gas chromatography technique. In the inspection of fish eyes in accordance with such a method, correct judgment can be achieved only when at least 20 to 30 fish eyes are examined.